The popularity of mobile data and voice communication continues to grow. The increasing popularity of data and voice communication requires that communication needs of a large number of users must be met, even in situations in which a large number of users are located within a small area, a case referred to as dense crowd scenario in the art. Typical examples include sports arenas, shopping malls or large office buildings.
In order to increase data transmission performance and reliability, the so-called multiple-input and multiple-output (MIMO) technology may be used in a wireless radio telecommunication system for transmitting information between a base station and a user equipment, for example mobile devices like mobile telephones, mobile computers and tablet computers and stationary devices like personal computers or cash registers.
MIMO systems may use multiple send and receive antennas for wireless communication at a base station as well as at the user equipment. The MIMO technology forms the basis for coding techniques which use the temporal as well as the spatial dimension for transmitting information. The enhanced coding provided in MIMO systems allows a spectral and energy efficiency of the wireless communication to be increased.
The spatial dimension may be used by spatial multiplexing. Spatial multiplexing is a transmission technique in MIMO wireless communication to transmit independent and separately encoded data signals, so-called streams, from each of the multiple transmit antennas. Therefore, the space dimension is reused, or multiplexed, more than one time.
If the transmitter is equipped with Nt antennas and the receiver has Nr antennas, the maximum spatial multiplexing order Ns (the number of streams or the rank) is Ns=min (Nt, Nr). This means that Ns streams can be transmitted in parallel, ideally leading to an Ns increase of the spectral efficiency (the number of bits per second and per Hz that can be transmitted over the wireless channel). For example, a MIMO system with a base station having two antennas and a user equipment having two antennas has a rank of 2 and is also called 2×2 MIMO, indicating the number of antennas at the base station and at the user equipment. However, the rank is limited by the device having the lower number of antennas, typically the user equipment.
In a so-called massive MIMO system, the base station may include a large number of antennas, for example several tens or even in excess of one hundred antennas with associated receiver circuitry. The extra antennas of the massive MIMO base station allow radio energy to be spatially focussed in transmissions as well as a directional sensitive reception which improves spectral efficiency and radiated energy efficiency.
In order to adapt the transmit signal at each individual antenna of the base station in accordance with the currently active receiving user equipment, a base station logic needs information about radio channel properties between the user equipment and the antennas of the base station. Vice versa, in order to adapt the transmit signal at each individual antenna of the user equipment, a user equipment logic need information about the radio channel properties between the base station and the antennas of the user equipment.
A pilot signalling scheme can be used for this purpose which allows the base station to set configuration antenna parameters for transmitting signals, so as to focus radio energy at the user equipment or for receiving radio signals from the user equipment. Likewise, the pilot signalling scheme can be used to enable the user equipment to set configuration antenna parameters for transmitting signals, so as to focus radio energy at the base station, or for receiving radio signals from the base station.
Thus, focus may mean both phase align contributions with different path length and transmit only in directions that will reach the user equipment and base station, respectively. In a conventional MIMO system, training sequences may be transmitted from all user equipment within the cell and possibly also neighbouring cells in a time slot which is dedicated to the respective user equipment. The training sequences need to be orthogonal in order for the base station to identify the configuration parameters for the plurality of antennas for each of the one of the user equipment in conventional systems. Orthogonality may be achieved by using time division multiple access (TDMA), code division multiple access (CDMA) or frequency division multiple access (FDMA) technologies or a combination thereof.
In case the MIMO system uses time division multiple access (TDMA), each user equipment can transmit a pilot signal in an assigned time slot, which can be received by the antennas of the base station and analysed by the base station logic. It will be appreciated that time slots are one example of orthogonal channels, with orthogonality being attained in the time domain. In order to not interfere with each other, a certain time period can be assigned in each system frame where each user equipment may transmit its pilot signal. The pilot signals may each include a training sequence with the pilot signal received at the plurality of antennas of the base station being analysed by the base station logic. Information about a radio channel property of the radio channel between the user equipment and the plurality of antennas may be obtained as a result of the analysis. The base station may use the results of the analysis to determine configuration parameters for transmitting signals via the antennas to the respective user equipment. Vice versa, the base station can transmit a pilot signal in an assigned timeslot, which can be received by the antennas of the user equipment and analysed by the user equipment logic to obtain a radio channel property of the radio channel between the base station and the antennas of the user equipment. The user equipment may use the results to determine configuration parameters for transmitting signals via its antennas to the base station.
Massive MIMO systems (MaMi) may be deployed in buildings such as office buildings, shopping malls, sports arenas or other areas in which a large density of users can occur. In such environments, a large number of user equipment devices may be located in a cell served by the MIMO base station. The time required for the pilot signalling of the user equipment in each frame may increase with the number of user equipment devices. For a large number of user equipment devices, the time required for all user equipment devices to transmit their pilot signals may exceed the available pilot signalling time in each frame. While the pilot signalling time, i.e. the number of time slots allocated to the pilot signalling, may be adjusted dynamically, the transmission of payload data would be negatively effected if the pilot signalling time was increased too much. Therefore, the resources for transmitting pilot signals are limited.
The pilot signals are send from the user equipment to the MIMO base station, i.e., in the uplink direction. Likewise, pilot signals may be sent from the MIMO base station to the user equipment, i.e., in the downlink direction. Therefore, uplink and downlink data transmissions are based on the quality of the uplink and downlink pilot signals. If there is interference during the pilot signal transmission, both uplink and downlink would be effected. The interference may originate from neighbour cells. Furthermore, for mobility reasons, the validity of the channel as defined by the configuration parameters is limited. A new pilot signal needs to be transmitted at regular terms, for example at about every millisecond. Therefore, the transmission of pilot signals requires a considerable amount of resources. In order to keep the ratio between payload and pilot signal overhead large, the number of orthogonal pilot channels needs to be kept as small as possible.